Scheduling of Multicast and Unicast Services under Limited Feedback by using Rateless Codes

Many opportunistic scheduling techniques are impractical because they require accurate channel state information (CSI) at the transmitter. In this paper, we investigate the scheduling of unicast and multicast services in a downlink network with a very limited amount of feedback information. Specifically, unicast users send imperfect (or no) CSI and infrequent acknowledgements (ACKs) to a base station, and multicast users only report infrequent ACKs to avoid feedback implosion. We consider the use of physical-layer rateless codes, which not only combats channel uncertainty, but also reduces the overhead of ACK feedback. A joint scheduling and power allocation scheme is developed to realize multiuser diversity gain for unicast service and multicast gain for multicast service. We prove that our scheme achieves a near-optimal throughput region. Our simulation results show that our scheme significantly improves the network throughput over schemes employing fixed-rate codes or using only unicast communications

Throughput of Rateless Codes over Broadcast Erasure Channels ABSTRACT

In this paper, we characterize the throughput of a broadcast network with n receivers using rateless codes with block size K. We assume that the underlying channel is a Markov modulated erasure channel that is i.i.d. across users, but can be correlated in time. We characterize the system throughput asymptotically in n. Specifically, we explicitly show how the throughput behaves for different values of the coding block size K as a function of n, as n approaches infinity. Under the more restrictive assumption of memoryless channels, we are able to provide a lower bound on the maximum achievable throughput for any finite values of K and n. Using simulations we show the tightness of the bound with respect to system parameters n and K, and find that its performance is significantly better than the previously known lower bound

Multiple access stability and broadcast delay in wireless networks

This thesis addresses issues of design and performance analysis in wireless communication networks. We investigate topics relevant to both uplink and downlink. For uplink, we study the stability region of the slotted Aloha protocol under the collision channel model, for the case of a finite number of independent users. The stability region (i.e., the set of arrival rate vectors such that the whole queueing system can be made stable) is in general unknown when the number of users is more than two. We seek to characterize the set of stabilizable rate vectors, whereas most existing works only provide bounds on the region of stabilized rate vectors under a given control (i.e., a vector of contention probabilities). We choose a natural and important inner bound on the exact Aloha stability region. The results we obtain include equivalent forms of and alternative membership testing for this set, as well as other properties such as various geometrically intuitive and simple inner and outer bounds, and generalized convexity properties of the associated "excess rate" functions. For downlink, we seek to characterize the delay when broadcasting (random linear combinations of) information packets over independent erasure channels to a finite number of users. Of interest is the random delay until all the receivers recover all the packets initially queued at the base station (i.e., the sender). This falls into the study of certain order statistic of random variables. We obtain lower and upper bounds, exact expressions and a finite-step computational procedure (recurrence) for the moments of the random delay. We also investigate the dependence of the delay on the code blocklength (under random linear combinations of packets as the scheme employed in random linear network coding), and on the number of receivers, respectively. Results here include asymptotics, monotonicity properties and asymptotically tight lower and upper bounds.Ph.D., Electrical Engineering -- Drexel University, 201